The invention pertains to electron multipliers and in particular, relates to electron multipliers having one or more micromachined channels and thin-film activation.
Conventional microchannel plate (MCP) manufacture relies on the glass multifiber draw process in which individual composite fibers consisting of selectively etchable core glass and a cladding glass are formed by draw down of a rod-and-tube preform. The multifiber bundles are stacked together and fused within a glass envelope to form a solid billet. The billet is then sliced, typically at a angle, with respect to the billet axis. The resulting wafers are polished and the soluble core glass is removed by a suitable chemical etchant to produce a wafer containing an array of microscopic channels. Further chemical treatments, followed by a hydrogen reduction process, produces a thin wafer of glass containing an array of hollow channels with continuous dynodes of reduced lead silicate glass having conductive and emissive properties required for electron multiplication.
The glass multifiber draw process, while commercially satisfactory and economical, suffers from a disadvantage that the size of the individual channels is governed at least in part by multiple glass drawing steps. Variations in fiber diameter can cause different signal gains within a microchannel plate and from one device to another. Another disadvantage is that the current technology allows for some irregularity in the array when multifibers are stacked and pressed to form the billet. Thus, there is no long-range order in channel location, and channel geometry is not usually constant across the array. There are other manufacturing difficulties involved in the various steps necessary to complete the device, a discussion of which may be found in U.S. Pat. No. 5,086,248, entitled Microchannel Electron Multipliers, by Horton et al., assigned to the assignee herein, the teachings of which are incorporated herein by reference.
Conventional fabrication of a channel electron multiplier (CEM) is simpler; it involves thermal working of lead silicate glass tubing into a curved or spiral geometry and reducing the glass surface in hydrogen to produce a continuous thin film dynode. Alternatively, an array of glass tubes can be bundled together and thermally worked into a helical configuration before activation of the channel walls by hydrogen reduction to form a multichannel device. Such manufacturing methods are labor intensive, and thus expensive, and can produce mechanical variability in the channel geometry which can affect electrical performance.
Attempts have been made to crystalize photosensitive glass in a lithographically defined pattern to render the crystallized region selectively etchable from the glass, leaving behind an array of channels for producing a microchannel plate. However, only moderate etch selectivity has been achieved and nonparallel sidewalls can result, limiting minimum channel spacing. Attempts have also been made to selectively wet etch a silicon wafer, however, simple holes with vertical sidewalls extending through the wafer cannot be achieved due to known crystallographic constraints.
Other approaches to the manufacture of the CEMs include the machining or molding of a ceramic substrate and activation with a reduced lead silicate glass or other thin film dynode (Carette and Bouchard Canadian Patent No. 1,121,858; Wolfgand U.S. Pat. No. 3,244,922; Fraioli U.S. Pat. No. 4,095,132; Schmidt and Knak U.S. Pat. No. 4,757,229). Fabrication of suitable ceramic substrates enclosing a curved or spiral channel is difficult and expensive.
In Horton et al. referred to above and Tasker et al., U.S. Ser. No. 08/089,771, entitled Thin-Film Continuous Dynode for Electron Multiplication, filed Jul. 12, 1993, and assigned to the assignee herein, the teachings of which are incorporated herein by reference, methods for selectively etching advanced technology microchannel plates and channel electron multipliers by use of a directionally applied flux of reactive particles along with the activation with thin-film dynodes is discussed. The present invention evolved from the need for structures to test dynodes for these advanced technology microchannel and channel electron multipliers formed by anisotropic directional etching and activated by thin-film techniques.
The disclosure herein describes micromachined electron multipliers of lower cost and greater ease of manufacture for detection of charged particles and energetic photons and a particular application for such devices in a photomultiplier tube. All of the illustrations are representative of the structure and arrangement and are not drawn to scale.